The use of polytetrafluorethylene or copolymers of tetrafluoroethylene as a printed circuit board substrate material or dielectric material is well known. The advantage of polytetrafluoroethylene include high temperature stability, low moisture absorption, and outstanding chemical resistance even at elevated temperatures. Additionally, the dielectric constant varies remarkably little over a wide range of temperatures and frequencies. This means that the use of these fluoropolymers results in the reduction of signal propagation delay, the reduction of signal attenuation, reduced distances between signal and ground planes for a given impedance value, and the reduction of crosstalk between closely spaced conductor lines. Thus, polytetrafluoroethylene and copolymers of tetrafluorethylene are uniquely suited for high-speed digital and high frequency printed circuit boards. In addition, reduced crosstalk makes possible higher circuit densities.
However, polytetrafluoroethylene and copolymers of tetrafluoroethylene also possess a relatively high coefficient of thermal expansion. A significant level of reinforcement is required to restrain this expansion as well as to provide mechanical durability to the finished printed circuit board. Failure to restrain this expansion can result in stress relief when the copper is selectively etched away to define a circuit pattern, leading to undesirably large dimensional changes in the printed circuit board. An appropriate level of reinforcement is reflected in good dimensional stability, or minimum dimensional change after the copper has been removed by etching and the unclad laminate has been thermally cycled. In general, a dimensional stability value of one mil per inch or less is considered to be to be good. This means that the average expansion or contraction of the material is only one mil per inch or less.
Polytetrafluoroethylene laminates are commonly reinforced with glass fibers. They can have good dimensional stability, but also have relatively poor electrical properties because the dielectric constant of glass (6.3) is significantly higher than that of polytetrafluorethylene (2.1). On the other hand, polytetrafluoroethylene laminates can be made with low glass levels to achieve good electrical properties at the cost of inadequate reinforcement and resultant poor dimensional stability.
It is also known to combine fluoropolymers with polyaramid fabrics and papers. Polyaramids are known to have high thermal stability and are usually combined with fluoropolymers to provide a composite that provides both chemical resistance and heat stability, as in Schmidt, U.S. Pat. No. 3,025,185, and Sasaki et al., U.S. Pat. No. 4,337,155. In some references the utility of such composites in printed circuit board applications is recognized. Hochberg, U.S. Pat. No. 3,136,680, discloses the use of polytetrafluorethylene reinforced with glass, asbestos, metal or other heat-resistant woven or nonwoven fabric in printed circuit boards. Obviously, the dielectric properties of the composite would vary radically depending upon which of the above reinforcing materials was used. Leibowitz, U.S. Pat. No. 4,513,055, discloses a controlled thermal expansion composite for use in printed circuit boards which comprises a fabric composed of yarn with a positive coefficient of thermal expansion and yarn with a negative coefficient of thermal expansion, which may be polyaramid, embedded in a resin matrix which may be a fluoropolymer. The level of resin used is generally about 50% by weight. Tokarsky, European Patent Application 0 178 943, discloses a para-aramid paper for use in printed circuit boards. Among many binders which can be used with the paper, fluorocarbon resins may also used where their special properties, e.g., low dielectric constant, low dielectric loss, and low moisture regain, are desired. However, it is well known that neither polyaramid nor fluoropolymer materials adhere readily to copper which is the most frequently used conductive layer in printed circuit boards. In many cases an adhesive layer must be used.
It is therefore the object of the present invention to provide a laminate which has both good dimensional stability and good electrical properties, and which provides good adhesion to copper.